This five-year proposal will develop methods to synthesize functional microvascular networks in vitro. Central to this work are: 1) the use of lithography to generate three-dimensional (3D) constructs that have open internal topologies in the shape of human microvascular beds, and 2) maturation of as-synthesized structures by sequential optimization of culture conditions (shear stress, lumenal pressure, oxidative stress, and matrix proteolytic activity). Conditions tailored for maturation of cylindrical microvessels, in which the flow profile is easily determined, will serve as a starting point for maturation of microfabricated channels and networks that have non-circular cross-sections. This work will provide culture conditions that enhance the development of two characteristic microvascular functions (barrier function and reactivity to cytokines) and will determine to what extent these functions may co-exist in vitro. Temporal correlation between the emergence of these behaviors and changes in histology will suggest strategies to further enhance maturation. This work will also determine how microvascular networks remodel (i.e., how their cross-sectional shapes and network topologies change over time), and will suggest design principles for 3D networks that preserve their initial topologies in the presence of continuous perfusion. More generally, this work will illustrate the unique capabilities of a lithographic approach to tissue engineering. In the short-term, these networks will serve as surrogate human vascular tissues for high-throughput screening of drugs that may perturb or enhance microvascular function. In the long-term, engineered networks may enhance the perfusion of artificial tissues in vitro, and thereby overcome one of the primary obstacles in tissue engineering.